This invention relates to a novel process for preparing 1,5-benzothiazepine derivatives represented by the formula: ##STR3## wherein one of R.sup.1 and R.sup.2 is a lower alkyl group or halogen atom, and the other is hydrogen, R.sup.3 is a lower alkyl group or a lower alkoxy group.
The above 1,5-benzothiazepine derivatives (I) are useful as an intermediate for the synthesis of, for example, the corresponding 3-acetoxy-5-[2-(dimethylamino)ethyl]-1,5-benzothiazepine derivatives having excellent hypotensive activity.
Heretofore, as a process for preparing 1,5-benzothiazepine derivatives (I), the method in which a propionic acid derivatives represented by the formula: ##STR4## wherein R.sup.4 represents hydrogen atom or an ester residue, and R.sup.1, R.sup.2 and R.sup.3 have the same meanings as defined above, is heated in a solvent (for example, xylene) to effect intramolecular ring closing has been known (U.S. Pat. Nos. 4,567,175 and No. 4,590,188). However, this method involves the problem of requiring a long period of time for the intramolecular ring closing reaction.